1. Field of the Invention
The present invention relates to an ink jet printing apparatus that forms an image by using a print head having a plurality of print elements each comprising a plurality of ink ejection nozzles arrayed in line. The present invention also relates to an ink jet printing method, a method of setting a print control mode, and a program.
2. Description of the Related Art
A printing apparatus of an ink jet system (ink jet printing apparatus) that ejects ink from nozzles arrayed in an ink jet print head to form an image on a print medium is finding a wide range of applications in such equipment as printers, facsimile machines and copying machines. A color printer capable of forming a color image using a plurality of color inks, in particular, is becoming increasingly popular as its print quality is enhanced.
In such ink jet printing apparatus, an increased printing speed as well as the enhanced print quality constitutes an important factor for their widespread use. An effort to increase the printing speed is being made, which includes increasing a drive frequency of ink ejection from the print head and using a greater number of nozzles arrayed in the print head. A technique currently available to dramatically enhance the printing speed, for example, involves elongating the print head and increasing a nozzle arrangement density to print in one scan an image that is otherwise printed in a plurality of scans.
Among the methods for elongating the print head, it is the most desirable in terms of production cost to arrange a plurality of print heads in line. More specifically, where each of the print heads is constructed of a chip having a plurality of nozzles, an elongate print head is formed by arranging in line the same number of chips as the print heads. In the following description a portion connecting the adjoining chips, each composed of a plurality of nozzles, is taken as a joint between the print heads.
At a portion of the printed image corresponding to the joint between the print heads, an image flaw that looks like a white line is likely to be produced. This is caused by a phenomenon in which an air flow produced between the print head and the print medium deflects ink droplets coming out of those nozzles at the ends of a nozzle column toward the inside of the nozzle column. As a result, the ink droplets fail to land where they are intended (this is also called an “end dot deflection”). Other possible causes for the stripe-like image defect include a difference in ink ejection volume among the print heads, a precision of arranging a plurality of print heads in line, and variations in time taken by ink droplets to land on the print medium.
To prevent such a stripe-like image flaw that occurs at a part of the printed image corresponding to the joint between the print heads, a method has been proposed, as in Japanese Patent Disclosure No. 5-57965, which overlaps the nozzles at the joint portion of the print heads.
In the ink jet print head, however, there is a possibility of ink droplets failing to be ejected normally (so-called “ejection failure”), which may be caused by dirt that enters into nozzles during manufacture, degradation of nozzles over the long period of use, and deterioration of ink ejection elements. If such faulty nozzles occur at the joint portion between the print heads, they in combination with the “end dot deflection”, the cause of the stripe-like image flaw, may produce more serious image impairments.
Even if the nozzles are not completely in the ejection failure state, the stripe-like image impairments such as caused by the ejection failure would likely occur also when the ink droplet ejection direction greatly deviates from an intended direction (also called an “excessive ejection deflection”) or when the ink droplet ejection volume differs greatly from the desired one (also referred to as an “ejection volume variation” or “drop diameter variation”). If nozzles in such an “excessive ejection deflection” state or “ejection volume variation” state should occur in the joint portion between the print heads, worse image impairments would result.
To realize both an increased printing speed and an enhanced print image, a method may be conceived that uses two print heads that eject inks of the same color and performs one print head scan to print at high speed with almost the same level of image quality that can be achieved with two scans (this method is referred to also as a “dual head configuration”). In this dual head configuration, if abnormal nozzles in the state of “ejection failure”, “excessive ejection deflection” or “ejection volume variation” should occur in one print head, the nozzles in the other print head that ejects the same color ink can complement the printing operation in place of the abnormal nozzles. However, if those nozzles of the second print head that are supposed to perform the complementary printing have troubles such as “ejection failure”, “excessive ejection deflection” or “ejection volume variation”, the desired complementary printing cannot be done.
The “ejection failure”, “excessive ejection deflection” and “ejection volume variation” of the abnormal nozzles have been able to be suppressed in the frequency of occurrence by improving the print head manufacturing environments and thus have not posed a serious problem. However, when two or more print heads are arrayed in line to increase the number of nozzles for faster printing speed, the “ejection failure”, “excessive ejection deflection” or “ejection volume variation” of the abnormal nozzles cannot be ignored. Efforts to produce print heads that do not include abnormal nozzles or which do not easily cause “ejection failure” will entail an increase in manufacturing cost, making the print heads very expensive.